PTL 1 discloses a technology related to improvement in performance of a de-duplication process executed in a storage system. When receiving write requests from clients, the storage system executes a first de-duplication process and executes a second de-duplication process at a subsequent appropriate timing. In the first de-duplication process, the storage system determines whether or not data to be written overlaps with any piece of data already written into a storage device (data ranked high among the data ranked in order of possibility to overlap with other data). When the data is determined to overlap, the storage system prevents the write data from being written into the storage device. Moreover, the storage system performs the second de-duplication process for data not yet evaluated if the data overlaps with the data already written into the storage device in the first de-duplication process, among the write data written into the storage device. Specifically, the storage system determines whether or not the data overlaps with the other data already written into the storage device. When the data is determined to overlap, the storage system deletes the write data written into the storage device or the data having the same data as the write data from the storage device.